Bootstrap 4 visual XS|SM|MD|LG|XL helper (place this code in start of body tag)

index.html

<!-- Adopt colors and sizes at your favor -->
<div style="position: fixed; font-size: 32px; background: #36f21c; left: 0; top: 0; z-index: 9999;">
   <span class="d-block d-sm-none">
     XS
   <br />
   </span>
   <span class="d-none d-sm-block d-md-none">
      SM
   <br />
   </span>
   <span class="d-none d-md-block d-lg-none">
      MD
   <br />
   </span>
   <span class="d-none d-lg-block d-xl-none">
      LG
   <br />
   </span>
   <span class="d-none d-xl-block">
      XL
   <br />
   </span>
</div>